The present invention relates to a feed water heater which is applied to thermal and nuclear power generation plants, and in particular, to a feed water heater which reduce low a solubility of an oxygen gas contained in a non-condensable gas generated during a heat exchanging process between a heating source and a source to be heated (heated source) to a steam condensing zone (drain collection zone).
In general, a feed water heater, which is applied to a thermal and nuclear power generation plant, has a structure, specifically, in which a turbine exhaust gas, which has performed expansion work in a steam turbine, is condensed into a condensed water (condensate) by means of a condenser, and then, when the condensed water is returned to a steam generator as a feed water, the condensed water makes a heat exchange with a turbine extraction steam so as to recover heat. For example, there is provided a conventional feed water heater having a structure shown in FIG. 9 to FIG. 11, in which FIG. 9 shows a horizontal sectional view of the feed water heater, FIG. 10 shows a cross sectional view taken along an arrow X--X in FIG. 9, and FIG. 11 shows a partially enlarged view of a portion XI in FIG. 9.
The feed water heater is composed of a semi-spherical water box 2 which is partitioned by a tubesheet (header) 1, and a transversely long cylindrical shell 3.
The water box 2 is partitioned by a partition (compartment) plate 4 and includes a feed water inlet 5 for guiding a condensed water supplied from a condenser (not shown) as a feed water, and a feed water outlet 6 for returning a feed water, which is heat-exchanged and pre-heated in the cylindrical shell 3, to a steam generator (not shown).
On the other hand, in the cylindrical shell 3, which contains U-shaped heat exchanger (transfer) tubes 8, as a plurality of tube banks, which is supported by the tubesheet 1 and a support plate 7, and a non-condensable gas vent tube 10 is provided for the central portion between the tube banks so as to extend axially and having suction ports.
Further, the cylindrical shell 3 is provided with a steam inlet 11 for introducing a turbine extraction steam as a heating source at its one side and an impingement plate 12 for reducing an impact stress of the turbine exhaust steam to the heat exchanger tube 8 at a position slightly separated from the steam inlet 11.
Further, the cylindrical shell 3 is formed with a steam condensing section 13 at the outer side of the heat exchanger tubes 8, and a drain cooling section 16 which is located beside the tubesheet 1 and is defined by a partition (compartment) plate 14. In the steam condensing section 13, the turbine extraction steam, which is introduced from the steam inlet 11 as a heating source, makes a heat exchange with a feed water flowing through the heat exchanger tubes 8, and then, a drain (condensed water) whose temperature becomes slightly low is gathered (on the bottom of the shell 3). On the other hand, in the drain cooling section 16, a heat is further recovered from the drain guided from the bottom of the shell 3 via a drain inlet 15.
In the drain cooling section 16, baffle plates 4 are alternately arranged so that the drain containing a bubble 17, which is entrained from the bottom of the shell 3 via the drain inlet 15, flows meanderingly. For this time, a retained heat of the drain is given to the feed water flowing through the heat exchanger tubes 8, and then, is supplied as a heating source for the feed water from a drain outlet 18 to, for example, another feed water heater.
As described above, the conventional feed water heater is provided with the steam condensing zone 13 and the drain cooling section 16 and is constructed in a manner that the heat of turbine extraction steam, used as a heating source, is almost thoroughly given to the feed water, and the heat is effectively used so as to improve a heat exchanging efficiency.
Recently, in the feed water heater, in the case of making a heat exchange of a turbine extraction steam used as a heating source with a feed water used as a heated source, concentration of an oxygen gas contained in a non-condensable gas concentrated in a steam has provided a problem.
That is, the oxygen gas contained in the non-condensable gas is partly dissolved in a condensing drain, and it is well known that the concentration of oxygen gas dissolved in the drain depend upon partial pressure of an oxygen gas according to Henry's law in the case where a temperature is constant. If the oxygen gas together with other non-condensable gas is left alone, the heat exchanging efficiency becomes worse, and the oxygen gas is dissolved in the drain with high concentration. This is a factor of corroding component members of the feed water heater. For this reason, in the conventional feed water heater, as shown in FIG. 9 and FIG. 10, the oxygen gas contained in the non-condensable gas concentrated during the heat exchanging process is gathered to the suction ports 9 of the non-condensable gas vent tube 10 located at the center of the heat exchanger tubes 8, and then, is discharged, with a certain amount of steam, to the outside from the feed water heater. This discharge rate is set within a range from about 0.5 to 2.5% of the turbine extraction steam supplied to the feed water heater, although it is different depending upon a type of plant.
However, according to recent research, the following matter has been found. That is, even if the non-condensable gas is gathered to the non-condensable gas vent tube 10 and is discharged to the outside of the feed water heater, during the heat exchanging process, the turbine extraction steam is condensed into a drain, and then, an oxygen gas of high concentration is still dissolved in the drain. The solution mechanism of oxygen gas to the drain will be described in more detail hereunder.
A solubility of the oxygen gas to the drain depends upon Henry's law in principle. However, if a bubble is mixed into the drain, the dissolved oxygen gas concentration rapidly becomes high. That is, if a bubble is mixed into the drain, the pressure of bubble rises up by a hydraulic pressure, and then, steam in the bubble is condensed into a drain rapidly due to cooling by the drain around the bubble. A partial pressure of the oxygen gas together with other non-condensable gas in the bubble rises up, and the solubility to the drain increases. As a result, the bubble becomes small, and the curvature of the surface of the bubble becomes large. For this reason, the pressure of bubble rises up more and more by the influence of surface tension, and the steam in the bubble is further condensed while the non-condensable gas being much dissolved in the drain. Finally, the bubble disappears or diminishes, and as a result, the drain contains an oxygen gas of high concentration.
The inventor of the present invention has carefully observed a change of drain taking the research result as described above into consideration. As a result, in the conventional feed water heater, as shown in FIG. 11, during the heat exchange with the feed water flowing through the heat exchanger tubes 8, a drain generated in the steam condensing section 13 flows into the drain cooling section 16 together with a bubble 17 via the drain inlet 15 and a clearance between the tube and the tube hole of the partition plate 14 supporting the heat exchanger tubes 8. Therefore, there is the possibility that a drain system after the drain cooling section 16 comes into contact with a dissolved oxygen gas of high concentration, and thereby, component members are corroded. For this reason, it has been desired to realize a feed water heater which can make low a dissolved oxygen gas concentration when the drain generated in the steam condensing section 13 flows into the drain cooling section 16 together with a bubble 17.